Scientists found regions on chromosomes 7, 8 and 10; Multiple genes - and not just the sex chromosomes - are important in sexual orientation, say US scientists.

A University of Illinois team, which has screened the entire human genome, say there is no one 'gay' gene. Writing in the journal Human Genetics, they said environmental factors are also likely to be involved. The findings add to the debate over whether sexual orientation is a matter of choice. Campaigners say equality is the more important issue.

Much of the past genetic research into male homosexuality had focused solely on the X chromosome, passed down to boys by their mother, according to lead researcher Dr Brian Mustanski. His team looked at all 22 pairs of non-sex chromosomes of 456 individuals from 146 families with two or more gay brothers. There is no one 'gay' gene.

Lead researcher Dr Brian Mustanski found several identical stretches of DNA that were shared among gay siblings on chromosomes other than the female X. About 60% of these brothers shared identical DNA on three chromosomes - chromosome 7, 8 and 10.

If it were down to chance, only 50% of these stretches would be shared, said the authors. The region found on chromosome 10 correlated with sexual orientation only when it was inherited from the mother. The most important thing is that lesbian and gay men are treated equally.

Dr Mustanski said the next step would be to see if the findings could be confirmed by further studies, and to identify the particular genes within the newly discovered sequences that are linked to sexual orientation.

"Our study helps to establish that genes play an important role in determining whether a man is gay or heterosexual," he said, but added that other factors were also important.

"Sexual orientation is a complex trait. There is no one 'gay' gene. "Our best guess is that multiple genes, potentially interacting with environmental influences, explain differences in sexual orientation."

Alan Wardle from the gay rights charity Stonewall said: "It's an interesting study that contributes further to the debate. "Regardless of whether sexual orientation is determined by nature or nurture or both, the most important thing is that lesbian and gay men are treated equally and are allowed to live their life without discrimination."

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